The invention is based on a priority application EP 03291778.3 which is hereby incorporated by reference.
The invention relates to a laser device for optical packet data transmission in burst mode system and to an optical transmitter in such a system.
Such laser devices and optical transmitters are basically known in the art. In particular, known laser devices comprise a loser light source for outputting a laser light beam and a laser driver for driving said laser light source. The known laser devices further comprise an optical modulator means for receiving the laser light beam output by said laser light source and a packet data source for outputting a packet data stream. Moreover, they comprise a modulated driver for receiving said packet data stream output by said packet data source and for driving or controlling said optical modulator means in response to said packet data stream resulting in that the optical modulator means modulates the laser light beam in response to said packet data stream.
The optical modulator means presently available on the market, suffer from the disadvantage that they are not able to completely close their light output even if they are driven by a shut-off signal, for example represented by a logical zero. This is because they only have a low extinction ratio. Consequently, there remains a residual light which is output by said optical modulator means, for example onto an optical fiber, even if an logical zero shall be transmitted.
If there is only one of these laser devices provided in a telecommunication system that residual light does not cause any difficulties in correctly detecting the transmitted logical value, in particular a logical zero, because a threshold value for detecting the transmitted logical zero might be adapted appropriately.
However, in dense wavelength division multiplex DWDM-systems being operated in a burst mode, a plurality of laser devices operate at the some wavelength and they all transmit the modulated laser beam via the same channel. Consequently, on the channel the outputs of all of said laser devices, more exactly of said optical modulator means are summed up. Thus, there occurs the problem, that if quite a lot of said laser devices or modulator means output said residual light, representing a logical zero, simultaneously, these residual lights are summed up on the channel and might be wrongly detected as logical “1” by a receiver receiving said laser beam.